The present invention is directed to an array of nanoresonators, and more particularly to an array of carbon nanotube resonators for use as a tunable RF filter.
Nanoscale structures are becoming increasingly important because they provide the basis for devices with dramatically reduced power and mass, while simultaneously having enhanced capabilities.
For example, mechanical resonators have been of significant interest because such resonators can exhibit orders of magnitude higher quality factors (Q) than electronic resonators. Such low-loss resonators are important for communications and mechanical signal processing applications. However, practical mechanical resonators have typically used bulk acoustic wave (BAW) oscillators, such as quartz crystals; or surface acoustic wave (SAW) oscillators. These bulk-crystal-based oscillators tend to be bulky and difficult to inexpensively integrate with high frequency electronic circuits. As a result, there has been a move towards Si-micromachined resonators, which can be monolithically integrated with conventional electronics. However, to date Si-based resonators have not been demonstrated at frequencies over 400 MHz, they suffer from reduced quality factors at higher frequencies, and they have limited tuning ranges. In addition, it appears that practical transduction mechanisms for the readout of Si resonators will be problematic for Si oscillators operating at or near the GHz frequency regime. Finally, data suggests that Si resonators will have a small dynamic range.
In contrast, nanoscale mechanical structures hold the potential to enable the fabrication of high-quality-factor (Q) mechanical resonators with high mechanical responsivity over a wide dynamic range. Such devices can form very low-loss, low-phase-noise oscillators for filters, local oscillators, and other signal processing applications. High-Q resonators are critical components in communications and radar systems, as well as in MEMS-based sensors such as a micro-gyroscope. The combination of high-Q with small force constants enabled by nanoscale resonators would also produce oscillators with exceptional force sensitivity. This sensitivity is important for a variety of force-detection-based sensors and may ultimately allow single molecule spectroscopy by NMR and optical techniques. (M. L. Roukes, Solid-State Sensor and Actuator Workshop Proceedings, Hilton Head, S.C., Jun. 4-8, 2000, p. 367.) Such mechanical oscillators are also key components for mechanical signal processing, which is of great interest because small-scale, high-Q mechanical elements may theoretically enable processing at GHz rates with orders-of-magnitude lower power dissipation than conventional CMOS processors. Arrays of nanoscale oscillators are of special interest because such arrays can store more energy than individual nanoresonators and so hold out the possibility of controlling and processing RF signals with significant power levels, as required for many practical communication and radar applications.
Despite the potential for these nanomechanical devices, the practical application of nanotube-based actuators and oscillators has been limited by the development of growth and processing methods for the control of nanotube placement and orientation. These techniques are critical for a wide variety of other nanotube applications including nanotube electronic systems.
One novel approach to making nanometer-scale structures utilizes self-assembly of atoms and molecules to build up functional structures. In self-assembled processing, atom positions are determined by fundamental physical constraints such as bond lengths and angles, as well as atom-to-atom interactions with other atoms in the vicinity of the site being occupied. Essentially, self-assembly uses the principles of synthetic chemistry and biology to grow complex structures from a set of basic feedstocks. Utilizing such techniques molecular motors have been synthetically produced containing fewer than 80 atoms. Chemical vapor deposition (CVD) appears to be the most suitable method for nanotube production for sensor and electronic applications. CVD uses a carbon-containing gas such as methane, which is decomposed at a hot substrate surface (typically 600-900 C) coated with a thin catalyst film such as Ni or Co. However, most studies to date have produced disordered nanotube films.
A notable exception is the work of Prof. Xu of Brown University who has developed a new technique for producing geometrically regular nanotube arrays with excellent uniformity in nanopore templates. Xu et al. Appl. Phys. Lett., 75, 367 (1999), incorporated herein by reference. Post-patterning of these ordered arrays could be used to selectively remove tubes in certain areas or produce regions with different length tubes. A variety of other studies have shown that dense, but locally disordered arrays of normally-oriented nanotubes can be selectively grown on pre-patterned catalyst layers.
In addition, there has been little progress in the control of nanotube orientation in the plane parallel to the substrate surface. Many of the basic electrical measurements of nanotubes have been done using electrodes placed on randomly scattered tubes after growth, or by physically manipulating tubes into place with an atomic force microscope (AFM). Dai and co-workers have been able to demonstrate random in-plane growth between closely spaced catalyst pads, including growth over trenches, as well as a related technique to produce nanotubes suspended between Si posts. Dai. Et al., Science, 283, 512 (1999), incorporated herein by reference. In these cases individual nanotubes sometimes contact adjacent electrodes by chance, and excess tubes can be removed with an AFM tip. This type of procedure can be effective for simple electrical measurements, but considerable improvements will be required for production of more complex nanotube circuits. Smalley""s group has demonstrated a wet chemistry-based method of control over nanotube placement using solution deposition on chemically functionalized substrates, although questions remain about nanotube length control and contact resistance. Smalley et al., Nature, 391, 59 (1998), incorporated herein by reference. More recently, Han and co-workers have demonstrated a process for lateral growth of nanotubes between two electrodes, but with uncontrolled nanotube location and orientation in the plane of the substrate (Han et al., Journal of Applied Physics 90, 5731 (2001)). None of the current techniques have been able to grow vertical individual nanotubes or small groups of nanotubes in controlled locations with integrated electrodes, as would be necessary to form nanotube oscillators or actuators.
At least one group has been able to produce vertical carbon nanofibers (V. Merkulov et al., Appl. Phys. Lett. 76, 3555 (2000)), but such nanofibers do not have the ideal graphite-like hollow tube structure of carbon nanotubes. Instead, the fibers exhibit a highly disordered xe2x80x9cbambooxe2x80x9d structure and will therefore have inferior surface and mechanical propoerties and be more suscpetible to losses that degrade resonator quality factors.
In addition to the problems associated with controlled growth and orientation of nanotubes, nanotube actuators and oscillators also require a transduction mechanism to convert input signals to physical motion and to provide corresponding output signals.
One possible electromechanical transduction mechanism is suggested by a recent demonstration that nanotube mats can serve as very high efficiency electromechanical actuators in an electrolyte solution, with the possibility of even better results for well-ordered single wall tubes. Baughman et al., Science, 284 1340 (1999), incorporated herein by reference. In brief, Baughman found that electronic charge injection into nanotubes results in a change in the length of the nanotubes. Conversely, it is expected that changing the length of a nanotube through an externally-applied force will result in the movement of charge on or off the tube, depending on whether the tube is stretched or compressed. However, this technique has only been demonstrated for large disordered arrays of nanotubes. Other potential actuation mechanisms include Coulomb force (electric field force on charges) interaction and light-induced nanotube motion. However, it is expected that light coupling to nanoscale-cross-section nanotubes will be inefficient.
There has been one report of high frequency resonator measurements on carbon nanotube bundles suspended over a trench in which Qs of around 1000 at 2 GHz were observed. (B. Reulet et al., xe2x80x9cAcoustoelectric Effects in Carbon Nanotubes,xe2x80x9d Phys. Rev. Lett. 85: 2829-2832 (2000)). In this case, Coulomb force coupling to a nonintegrated macroscopic local antenna was used to drive the mechanical resonance of the individual suspended nanotube bundle. However, the carbon bundle resonator required superconducting electrodes and low temperature measurement. Furthermore, the individual nanotube resonator is not suitable for RF power handling operation because the vibrational mode of such a device can store and transmit very little energy.
Accordingly, a need exists to develop nanoscale mechanical devices, such as, resonators to enable real-world applications ranging from molecular-scale characterization to ultra-low-loss mechanical filters and local oscillators for communications and radar, to rad-hard low-power mechanical signal processors.
The present invention is directed to a tunable nanomechanical resonator system comprising an array of nanofeatures, such as nanotubes, where the nanofeatures are in signal communication with means for inducing a difference in charge density in the nanofeature (e.g. by biasing one end of the feature) such that the resonant frequency of the nanofeature can be tuned, and where the nanofeature is in signal communication with a waveguide or other RF bias conduit such that an RF signal having a frequency corresponding to the mechanical resonant frequency of the array will couple to the array thereby inducing resonant motion in the array of nanofeatures, which in turn couples to an RF output waveguide structure, forming a nanoscale RF filter. The invention is also directed to growth techniques capable of producing a nanoscale RF filter structure controllably positioned and oriented with a waveguide and integrated electrodes.
In one embodiment, this invention utilizes an array of carbon nanotubes with an integrated electrode on a substrate that functions as an RF filter. This invention is also directed to a device, which utilizes multiple arrays of carbon nanotubes with integrated waveguides and electrodes that functions as an RF filter bank. This invention is also directed to novel systems and methods for utilizing devices comprising at least one array of nanotubes with an integrated waveguide and electrode.
In another alternative embodiment, the nanotube array oscillation is produced by optical irradiation or by direct RF coupling to the array via base and top electrodes.
In still yet another embodiment, the invention is directed to growth and processing techniques to control array location and orientation; and methods for positioning nanotubes during growth, including nanoscale patterning of the substrate to ensure that the growth of the nanotubes is ordered and oriented properly with the waveguide and integrated electrodes.
In still yet another embodiment the nanotubes comprising the arrays are self-assemble into arrays in which each nanotube has a specified diameter and height suitable for use in the devices of the current invention.
In still yet another embodiment, the substrate is made of a semiconductor such as, for example, oxidized silicon or aluminum oxide, coated with a metal catalyst film such as, for example, Ni or Co. In this embodiment, the silicon can be further doped to adjust the electronic properties of the substrate surface.
In still yet another embodiment, the nanotubes comprising the arrays are self-assembled from an inert material such as, for example, carbon utilizing a carbon feedstock gas such as, for example, ethylene.